Glass compositions and glass-to-metal seals



. 2,931,100 Z GLASS comosmogg A lgn GLASS-TO-METAL Lucy Florence Oldfield, Harrow Weald,,England, John Henry Partridge, deceased, late of Pinner, England, by

Ethel Partridge and Michael John Anthony Partridge,

" administrators, Pinner, -England, assignors to The Gen- -'eral Electric Company Limited, London, England, a

British company No matting; Application May 13, 19'57 Serial No. 658,530

Claims priority, application Great Britain March 17, 1953 6 Claims. or. 106 .53)

This invention relates to electrolyte solution. It has been found that the resistance to weathering of glasses of this type can be improved by increasing the alumina content, but this results'in an increase in the difficulty of founding the glass; order to overcome these defects, the boric oxide content of the glass is reduced, the relative proportions of the remaining constituents being simultaneously increased, the coefiicient of thermal expansion of the resulting glass is too high for forming satisfactory seals to iron-nickelcobalt alloys of the kind referred to above. l r v It is'an object of the present invention to provide a range of glasses which are suitable for sealing to metals and alloys of the type referred .to, and which are less' subject tothe disadvantages mentioned in the preceding paragraph.

The discovery underlying this invention is that this object can be achieved by simultaneously reducing the tively, or a linearthermal expansion characteristic in the range 20 C. to 550 C. such thatthe mean coefiicient of thermal expansion over this temperature'range is inthe range 5.3 to'5.7 l0- cm./cm1./. C. The invention also relates to glass-to-metal seals obtained by sealing these glasses to such metals and alloys. Glass-to-metalseals of the, kind with which the invention is concerned may be used, for' example, in the manufacture of certain electrical devices, for sealing a glass part of the envelope of the device to a metal part, or for sealing metal conduct-' ing members through a glass wall of the envelope. Alloys having the non-linear thermal expansion characteristics referred to, above the kindcomposed of the-metals iron, nickel and cobalt, in the approximate proportions of 54% iron, 29% nickel and 17% cobalt. A metal having a linear thermal expansioncharacteristic of the kind referred to is molybdenum.

For forming satisfactory glass-to-metal seals, the thermal expansion characteristics of the glass and the metal or alloy used must substantiallymatch, that is to say, the thermalcxpansion curves of the glass and metal should not be very diiferent from each other; the extent to which the thermal expansion curves can differ in any particular case} will, of course, depend on the design of the sea% and the purpose for which it is required.

has previously been proposed to use glasses of high bo 1c oxide and low alkali and alumina contents, that is t ay, containing at least 22% boric oxide, .less than 1W total alkali metal oxides, and about 2% alumina for scaling. to metals of the types specified, since these glasses have the required thermal expansion characteristics. These glasses, however, are liable to present certain difiiculties in manufacture as a result of the high boric oxide content; the boric oxide is liable to volatilise to some extent during the glass founding operation, resulting in the production of glasses of different physical characteristics from different batches of the same initial composition. Further, the high boric oxide glasses are not amenable to the fabrication of bulbs and tubing of large sizes, because they tend to develop cord, which is difficult to remove, during founding. These glasses also suffer from the disadvantage that they are susceptible to attack by acids which may be used for washing glassto-metalseals in manufacture, and that under conditions of high humidity their surfaces trically conducting, due to the formation of a film of an include, for example, alloys of fluorine, barium oxide, and

boric oxide content and increasing the alumina content of the glass composition, boric oxide glasses mentioned above, and introducing optionally lead oxide'fand/or zinc oxide. i i According'to the present invention, therefore, a glass suitable for sealing directly to a metal or alloy of the type specified has a composition 'in the rangeof 64% to.68% silica (SiO ),'3.5% to 6% alumina (A1 0,), 12% to 16.5% boric oxide (B 0 3% to 5% sodium o'xidetNa 'o), 2% to 5% potassium oxide (K 0), 0% to.1.5% calcium oxide (CaO), 0% to 2.5% zinc oxide (ZnO), 2.5% to 3.5% vbarium oxide (BaO), 0% to 7% lead oxide (PbO), and 0.2% to 2% content of alkali metal oxides (Na O+K O.) being from 6% to 8% and the total content of divalent metal oxides being, from 4% to 8%. i .All proportions referred to in this specification are by weight, and the range of compositions referred to above is inclusive of the end figures quoted for each component. It is to 'be noted that the compositions included within the above statement of the invention refer to the composition'of the glass after founding. It will be appreciated that ananalysis of a glass will in most cases show that its actual composition differs slightly in some respects from the composition calculated from the starting materials used, as a result of changes in composition which may occur during founding due to the loss of part of one or more constituents by volatilisation or to corrosion, by the molten glass, of the refractory material of which the glass-melting pots or tank furnace walls are formed, resulting in the incorporation of sinall proportions of said material in the glass. It will also be appreciated that, for obtaining glass of a composition as close. as possible to the required composition, it will be necessary when preparing the initial batch of raw materials, to take into account such changes in composition which may occur during founding land to adjust the composition of the starting batch accordingly, in convert tional manner. Thus, as is well known to occur in glass founding operations part of the contents of fluorine, alkali metal oxides and boric oxide in the batch are liable to be lost by volatilisation at the glass-melting temperatures used in the production of the glasses of the present invention, and therefore it is usually desirable to include in the batch correspondingly larger proportions of these constituents thanv those included in the nominal composition of the glass which it is desired to produce. Furthermore small amounts of alumina are usually removed from the pot or furnace walls by the molten vglass and incorporated in the glass, so that to compensate for this gain in alumina it is necessary to reduce the pro portion of the alumina-producing constituent in the batch, in accordance with usual practice in glass manufacture. For the manufacture of a glass in accordance with the;

Patented May 17,

If, in"

as compared with the fluorine, the total the batch may be made up of any commonly used'in the glass-making industry for providing the various constituent oxides of the glass. For example, in addition to sand the batch include sodium nitrate, potassium carbonate, borax and/ or boric acid or boric oxide, barium carbonate, and optionally zinc oxide and lead'oxide; cryolite may be employed as a source of fluorine and aluminium, or the fluorine may be derived from sodium silicofiuoride while the alumina is provided by some other common aluminacont'aining mineral such as feldspar; alternatively, if calciurn oxide is to be included in the final glass composition, calcium fluoride may beemployed as the source of fluorine. If desired a small agent such as arsenious oxide may be included in the batch: the amount of such material used is usually so present invention, suitable ingredients s'mallas to be discernible only as a trace in the final product. The latter might also contain trace impurities resulting from impurities in the starting materials used, but these should, of course, never be allowed to be so high as to detract from the desirable properties of the glass.

T he proportions of those ingredients of the batch which furnish the alkali metal oxides, boric oxide, alumina a'nd'fl'uorine, may be adjusted as necessary to compensate for losses and gains during founding, as indicated above. The batch'adjustmentsrequired will depend to some extent upon the temperature at which the glass is founded and upon the nature of the material of which the glassmelting pot or tank employed is formed. 'By carrying out the founding under carefully controlled temperature conditions, and in a pot or tank composedof material not'subject to appreciable corrosion by the molten glass, it is possible to produce a glass having'an analytical composition very close to thenominal composition, with little adjustment of the batch composition. The glasses of the invention m'ayb'e founded under conventional conditions, the temperature employed usually being between 1400 C. and I500 C., and the founding of each batch being completed in a period of 1 /2 to 2' days, for quantities of /2 to 1 ton, but of course shorter periods, for example 4 to 6 hours, only are required for small quantities, for example '5 lbs. or less.

' The thermal expansion characteristics of the glasses in accordance with the present invention which we have measured have been such that over the temperature ranges 20 C. to 350 C., 20 C. to450 C., 20 C. to 500 C., and20 C. to 550 C. respectively, the mean coefficients of thermal expansion'were in the ranges 4.7 to 5.3, 5.0 to 5.4, 514 to 6.3, and 6.0 to 7 .0 l"' cm./cm./ C., respectively; such glasses are sufficiently well matched in respect of' their thermal expansion characteristics, to the aforesaid iron-nickel-cobalt alloys and to metals such as molybdenum, to be suitable for the manufacture of glass-to-metal seals with these alloys and metals formost'purposes. The aforesaid glasses also had highv electrical resistivities, of the order of 10 to 10 ohm-cm. at 150 C., which is an advantageous property in glasses to be used for glass-to-rnetal seals in the manufacture'of electrical devices.

The glasses according to the invention possess-various advantages as a result of the substantial reduction in boric oxide content. They are founded more easily to produce glasses which can readily befabricated into ware of" good qualityfro'm the point of'view of homogeneity of the glass, which'may be obtained substantially free from cord, stones and other defects. The reduction in the boric oxide content also results in a decrease in the effect of humidity and attack by the dilute mineral acids which are generally used for washing glass-to-metal seals in cases where the metal is welded after sealing. The increase in the alumina content also increases the resistance to weathering and to acids. The introduction ofiluorine, barium oxide and lead oxide not only counteracts the effect of the reduction of the boric oxide proportion of a'known fining content on the coefficient of thermal expansion of the glass, but these additions also soften the glass,thus facilitating scaling to metal, and reduce the high temperature viscosity of the glass, hence enabling founding to take place more rapidly, with more rapid removal of .seed or gas bubbles. The presence of'lead oxide is not objectionable in the formation of glass-to-metal seals, and has an additional advantage inthat it further increases the resistance of the glass to weathering and chemical attack. The increased resistance to the effect of humidity at all temperatures enables these glassesto be used as insulators for electrical components when'isealed to irdnnickel-cobalt alloys of the,aforesaidkind,folexample, without any danger of loss of efficiency as insulators caused by surface conduction throughiilmsof moisture containing dissolved'salt's'derived from the glass, such as is experienced with electrical components using the high boric oxide type of glass as insulators. 1

The glasses in accordance with the invention are particularly suitable for the fabrication of ware, such as bulbs or tubular components of all sizes for forming parts of the envelopes of electrical devices, for example thermio'nic valves and semiconductor devices, which parts can readily be sealed directly to envelope parts composed of a metal or alloy of the type specified. For example a tubular seal of any required diameter may be formed by placing the open end of a bulb or tube composed of one of said glasses in contact with a disc or tube of such metal or alloy and heating the contacting parts of the glass and metal components to about 1000 C. by any convenient method. These glasses are also suitable for the fabrication of prismatic lighting ware, for example for use in street lighting fittings and in sealed beam headlights.

The glasses of the invention may furthermore be used to form glass beads for sealing leading-in conductors into apertures in metal parts of the envelopes of electrical devices, the beads being produced in well known manner either by sintering the compressed glass powder or by fusing on to the leading-in wires short lengths of drawn glass tubing fitted over the wires.

The compositions of two specific glasses in'accordance with the invention are shown in the following table, in columns B and C, by way the composition of a typical high boric oxide glass (column A) which maybe used for sealing to alloys of the Kovar type, for comparison; the figures quoted in the table represent weight percentages. Themean coefficient of thermal expansion over the temperature range 20" C. to 350 C. (a), the Mg point (that is 'to'say, 'the highest. temperature attainable on the thermal expansio'mtemperature curve, above which temperaturetheglass'softens at a measurable rate under its own weight), and the electrical resistivity inohm-cm. at C. and 25 C. (indicated as log 10p) of each glass are also indicated in the table.

Table it s l o 1.9 0.3 1.5 5.3 5.0 Mg C.) 25 560 550 teem, at 150 O. .1 '11. 2 11, i log), at 25 C .9 .16. s is. s

As an example of a methodofmanufacturing a glass in accordance with the invention, a typical batch of'raw of example, together with materials is prepared by in the weight proportions stated:

i a. 19 Barium carbonate 6% Zinc oxide 4 Felspar I I 70 Sodium nitrate 2 Dehydrated borax. 22 Boric acid. 36 Sodium silicofludridefil; 2 Potassium carbonate l." 3%

A small amount, for. example about 6 ounces-,iof a fining agent such as arsenious oxide might also be included in the batch.

Approximately half a tonof batch of theabove composition is introduced into an open fireclay pot 45' inches in diameter, is heated by either oxy-coal gas burners or fuel oil burners to a temperature of 1430 C. and is maintained at this temperature for 1% to 2 days. The molten glass is allowed to cool to a convenient working temperature for the fabrication of ware, which is about 1100-1l50 C.; cooling from the founding temperature to the working temperature takes about 3-6 hours. The composition of the glass thus produced, allowing for loss by volatilisation of part of the fluorine and oxides of sodium, potassium" and boron contained in the initial batch and pick-up of alumina from the wall of the pot, is as follows:

. Percent by weight SiC), 65.5 A1 5.7 B203 15.7 Na o 4.4 K 0 3.1, ZnO 2.0 BaO 2.8 F, 0.2

The glass may also contain a small proportion or 0210 derived from the felspar and possibly traces of other minor impurities from the raw .materials, and a small proportion of the, fining agent, ifus ed, but these minor aiiditions do not aflect the quality or properties of the g ass.

If loss by volatilisation proves .to be greater or less than expected, the proportions of borax, boric' acid, sodium silicofiuoride and potassium carbonate included in the batch may be adjusted as required; for example, in various batches which we have employed, the weights of dehydrated borax and boric acid respectively have ranged from 16 to 22 lbs., and from 28 to,42 l-bs., the weights of the other ingredients remaining the same as indicated above.- Similarly, the amount of sodium silicofluoride used may be varied between 1% to2 /fi lbs, and the amount of potassium carbonate may be varied between 3 to 4 lbs. In addition, the amount of sodium nitrate employed may be varied, :for example between about 1% lbs. and 3 lbs., primarily seed in the glass, although this will of course have some effect on the sodium oxide content of the glass.

The glass produced by the method described above, and also" glasses. B and C referred to in the above table, are suitable for the fabrication of bulbs and tubes of widely varying sizes, by conventional glass blowing and drawing techniques, and seal directly to iron-nickel-cobalt alloys of the kind hereinbefore mentioned. Hence these glasses can advantageously be used for the manufacture of glass of the envelopes of many types of electrical devices, for "example transmitting.valve'envelopes, mercury arc rectifier envelopes, and germanium diode capsules. In addition these glasses may be used, in the form of beads, for sealing conducting wires into apertures in metal comof electrical devices, the beads being formed ponents either {rem short lengths of drawn tubing, or-trom sinthe renewing ingredient-t,

for the purpose of controlling 40 meshes to the linear inch but retained-by a sieve of glass components or parts of which metal components of such devices are united 200 meshes to the linear inch. powder is mixed with approximately 5% of its weight of a suitable binder, for example cetyl alcohol in solution in ether, and the mixture is dried and then subjected to a pressure of 4- to 5 'tons persquare inch in a'mould "to form beads' of the*required size and shape having apertures through which the conductors can be inserted. U The-heads are sintered by heating at 725C. a

For the manufacture of allthe above-described types of seal, the glass and metal com nents are assembled 'to,- gether in the appropriate relative positions and are heated, in the regions adjacent to the required seal, to approximately 1000" C. in known manner, for example but direct flame heating or by the eddy current technique. The seals are annealed at 525 C.

The glasses in accordance with the invention may also be used in the .form of loose powder, to form pressed of electrical devices, by means together. As an example of such a use, a pressed glass base of an electrical device having a plurality of leadingin-wires, composed of an iron-nickel-cobalt alloy of the kind referred to, passing through the base is made in the following manner: the glass produced by the method described above by way of example, in the term of loose powder produced as described abovein connection with the manufacture of sintered glass beads, is placed, without total content of barium oxide (BaO),

a binder, in a graphite mould of the shape and size of the required base, and the wires are placed in the desired positions, passing through the mass of glass powder, located by apertures in the mould. The'mould is subjected to light'pressure, therein is raised to 900 C. byeddy current heating. The perimeter of the glass base so produced may be sealed directly to an envelope or canister formed of an ironnickel-alloy similar to that of which the leading-in Wires are composed, by heating to approximately 1000 C. as described above.

'Ihisapplication is a continuation-in-part application of earlier filed application Serial No. 416,394, filed March 15, 1954, and now' abandoned, for Glass Compositions and Glass-to-Metal Seals.

There is claimed:

1. A lama-nee glass, suitable for sealing directly to 3.5% to 6% alumina (A1 0 12% to 16.5% (B 03), 3% to 5% sodium oxide (Na O), 2%.to 5% potassium oxide (K 0), 0% to 1.5% calcium oxide (C), 0% to 2.5% zinc oxide (ZnO), 2.5% to 3.5% 0% to 7% lead oxide (PhD), and 0.2% to 2%'fluorine, the total content of alkali metal oxides (Na 0-FK 0) being from 6% to 8% and the divalent metal oxides being from 4% t0 8%. j v I 2. A lithia-free glass having the composition, by weight, which consists of approximately 65% SiO 5.7% A1 0 15.8% B 0 4.7% Na O, 3.2% K 0, 0.9% CaO, 2.5% BaO, 1.9% ZnO and 0.3% fluorine, and of which the coefficient of thermal expansion is 5.3! I0- cm./cm./ C. over the temperature range 20 C. to 350 C., the Mg point is 560 C., and the electrical resistivity is 10 ohm-cm. at C. and 10 ohm-cm. at 25 C.

3. A lithia-free glass having the composition, by weight,

while the temperature of the glass composition essentially I consisting of in the range of 64% to 68% silica (SiO boric oxide,

which. consists. of approximately 66.5% SiO-z, 4.Q% Al Q 14.0% B 410% N3 0, 2.5% K 0, 3.0%Ba0, 4.5% Pb( 1.5% fluorine, and of which the coefficient of thermalexpansion is 5.0 10-' cm./crn./ C. overthe temperature range 20" C. to 350 C., the Mg point is 550 C., and the electrical resistivity is 10 ohm-em. at 150 C. and10 ohm-cm. at 25 C.

4. A lithie-freeglass havingthe composition; by weight, whiehconsists of approximately 65.5% SiO 5 .7 A1 0 15.7% B 0 4.4% N3 0, 3.1% 0, 2.;0% Z110. 2.8%

0 nd 01 2% n,

5- A a Svto-nlfltfl seal e ist n fi e glass aewrsi ng to elaim ,1 sea e d rectly to an a loy h n "a n nieear herm l xpansion eha ae e ist e in th tempera urer -nge to .5 0 C,- uch th t ver the t mp at re ranges 20.? 9.110359" C.-,-.20 C o 50 ,20 Q @15 0 C and 20 C. to'S 50 C., respectively, the rnean ceeflicients of thermal expansion are approximately 4.8, 5 .3, 6.2,

i 6. A gl as s-to-metal seal consisting of a glass according to claim 1 sealed directly t thermal expansion characteristic in the temperature range C. to- 550 C. such that the mean coeflicient of in the range 5. 3 to thermal expansion over this range 13 5.7 '10- cm/cmy C.'

' References Cited in the file of this patent UNITED STATES PATENTS 2,062,335 Scott Dee. 1, 1936 2,068,801 'Hood Jan. 26, 1937 OTHER REFERENCES Gla stechnisehe"Tabellen, Eitel-Pirani-Scheel, 1932 pages 689 and 691. 7

o a metal having v,a linear July 24, 1956 

1. A LITHIA-FREE GLASS SUITABLE FOR SEALING DIRECTLY TO A METAL OF THE CLASS CONSISTING OF MOLYBDENUM AND IRONNICKEL-COBALT ALLOYS, HAVING A COMPOSITION ESSENTIALLY CONSISTING OF IN THE RANGE OF 64% TO 68% SILICA (SIO2), 3.5% TO 6% ALUMINA (AL2/3), 12% TO 16.5% BORIC OXIDE (B2O2), 3% TO 5% SODIUM OXIDE (NA2/), 2% TO 5% POTASSIUM OXIDE (K2/), 0% TO 1.5% CALCIUM OXIDE (CAO), O% TO 2.5% ZINC OXIDE (ZNO), 2.5% TO 3.5% BARIUM OXIDE (BAO), O% TO 7% LEAD OXIDE (PBO), AND 0.2% TO 2% FLUORINE , THE TOTAL CONTENT OF ALKALI METAL OXIDES (NA2O+K2O) BEING FROM 6% TO 8% AND THE TOTAL CONTENT OF DIVALENT METAL OXIDES BEING FROM 4% TO 8%. 